This invention relates to epoxy resin compositions. In one aspect, the invention relates to the curing of epoxy resin compositions.
Cationic curing agents for epoxy resins, such as Lewis acids (BF.sub.3 and SnCl.sub.4, for example) and their complexes, protonic acids containing anions of low nucleophilicity (HBF.sub.4, HPF.sub.6, HSbF.sub.6, CF.sub.3 SO.sub.3 H and H.sub.2 SO.sub.4, for example) and certain salts of these acids have a number of useful properties. Many of these curing agents will cure liquid epoxy resins to hard solids without application of external heat within seconds or minutes after the resin and curing agent are mixed at room temperature.
Many of the most rapid curing agents for epoxy resins, such as HBF.sub.4, HPF.sub.6 and a number of tetrafluoroborate salts, are available commercially only as aqueous solutions. These aqueous solutions are not soluble in bisphenol-A based epoxy resins and form suspensions of small globules when mixed with the resins. Cure of the resins is dependent on the diffusion of the cationic initiator from the globules of aqueous solution into the bulk of resin phase. The rate of cure of the resin will therefore be strongly dependent on the size and degree of dispersion of the globules of aqueous curing agent solution and hence on mixing intensity. In some cases, rapid cure can occur at the interface around a particle of aqueous curing agent solution, which retards or prevents diffusion of the curing agent into the bulk of the resin and results in delayed or incomplete cure.
One approach to the problem would be using organic solutions of the cationic curing agents rather than aqueous solutions. This generally improves mixing for those curing agents which are soluble in organic solvents. Some cationic curing agents, however, are not very soluble in organic solvents. Others, such as HBF.sub.4, are unstable in some organic solvents, tending to revert to gaseous HF and BF.sub.3. Even for soluble curing agents, however, the evaporation of water from an aqueous cationic curing agent solution and its replacement with an organic solvent requires considerable energy expenditure and inconvenience.
It would therefore be desirable to provide a method for improving the cure reproducibility of epoxy resin systems containing aqueous cationic curing agents.
It is therefore an object of the present invention to provide novel epoxy resin compositions containing cationic curing agents. It is a further object to improve the cure rate reproducibility of epoxy resin systems containing aqueous solutions of cationic curing agents.